<!--
 * @Author: zengwj
 * @Date: 2022-08-31 17:17:57
 * @LastEditors: zengwj
 * @LastEditTime: 2022-09-02 11:18:55
 * @Description: 
-->
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box {
      width: 200px;
      height: 200px;
      border: 10px solid red;
      padding: 20px;
      margin: 20px;
    }
  </style>
</head>

<body>
  <div class="box"></div>
  <script>
    let a = ''
    const len = a.length;
    a = a.substring(len - 5, len);
    console.log(a)
  </script>
</body>

</html>